Manhole cover manufacturing process and manhole cover thus obtained

ABSTRACT

A process for manufacturing a manhole cover having a thickness of at least 10 mm, and up to 80 mm, comprising the generation of a mixture of reagents able to generate a polymeric thermosetting material, the mixing of suitable reinforcement fibres, the introduction into an open mould of the mixture thus obtained, the closure of the mould with a suitable counter-mould, the hardening of the said mixture, the opening of the mould, and the removal of the manhole cover obtained. The process makes use of the co-injection of reagents for the generation of the polymeric thermosetting material and the reinforcement fibre. The present invention also describes a manhole cover obtained with the said process.

The present invention relates to a manhole cover having a minimum thickness of 10 mm, in particular a manhole cover able to be subjected to bending loads, even high loads, in particular a manhole cover for pits, such as drain shafts, manholes, and suchlike, able to close an access opening, which may be located, for example, in the ground or a floor, and able to bear heavy loads, such as those transmitted by the wheel of a vehicle that comes to rest on the manhole cover, while the latter is supported around its edges only. The invention also concerns a process for the production of the said manhole cover.

Manhole covers have long been made of metallic materials, such as steel or cast iron. Because of the ensuing great weight, as well as for corrosion resistance reasons, attempts have been made to replace the metallic material with lighter, cheaper materials, such as thermosetting plastics, particularly polyurethane. Among other things, these materials offer the advantage of allowing the manufacturing of pieces that are less noisy when installed and subjected to the passage of vehicle wheels. To ensure high mechanical resistance, particularly during bending, it has always been necessary to provide reinforcement structures to embed in the plastic material during moulding of the article. For example, structures have been proposed such as a suitable intertwined pultruded fibreglass rods to insert into a mould prior to the injection of a plastic material. The fibreglass can be rendered compatible with the plastic material utilised by means of coatings with appropriate chemical agents. Plastic materials utilised include polyester and polyurethane, the latter being preferable, due to its resistance to wear, for articles that are exposed to the external environment, without any kind of protection.

The need to insert the structure into the mould makes manufacturing operations longer and more complex. Problems are also generated by the inhomogeneous structure of the article produced, which is composed of volumes of materials which are completely mutually different, with completely different mechanical behaviour. The non-random orientation of the rods means that they can explicate their pulling action in a precise direction, therefore the structures typically include multiple layers of rods with different orientations, which can be mutually joined by fibres or other adhesives to render positioning precise. To keep the structures in the correct position, closed moulds are generally used, into which the polymer mixture is subsequently injected. This renders it more difficult for the polymer mixture to penetrate all the empty spaces in the mould, with the risk of formation of bubbles. A critical factor, furthermore, is the penetration of the mixture into the internal spaces in the structure, which must be designed also taking into account this aspect of the process.

It would therefore be desirable to obtain a manhole cover made of a plastic material with adequate mechanical resistance properties, by means of a simple and inexpensive process.

The aforesaid problems have now been overcome by means of a process for manufacturing a manhole cover having a thickness of at least 10 mm, comprising the generation of a mixture of reagents able to generate a polymeric thermosetting material, the mixing of appropriate reinforcement fibres, the introduction in an open mould of the so obtained mixture, the closure of the mould with a suitable counter-mould, the hardening of the said mixture, the opening of the mould and the removal of the manhole cover obtained.

The invention also relates to a manhole cover made of a thermosetting plastic material, having a thickness of at least 10 mm, comprising a plurality of reinforcement fibres (meaning that a layer with a different fibre concentration may be envisaged, as may be a substantially fibre-free layer), which is uniformly dispersed in the volume of the thermosetting material, in particular a manhole cover obtainable by means of the aforesaid process.

The manhole cover is, in particular, a manhole cover able to withstand the application of a load of at least 100 kN, applied—in accordance with the testing procedures envisaged by standard EN124—to the upper face, with the manhole cover laid and the border (measuring a minimum of 5 mm) around the edge of the lower face resting in position.

Preferably, the thermosetting plastic material is a two-component material, usually comprised of two different monomers. Examples thereof are polyester or epoxy resins. Particularly preferred is polyurethane, with the mixture of reagents preferably comprising a polyol and an isocyanate of an appropriate type. Preferably, both components have a viscosity of no more, than 5000 mPa·s, and more preferably no more than 3000 mPa·s, in the absence of mineral fillers. If mineral fillers (e.g. calcium carbonate) are employed, the viscosity of the components after addition is preferably no more than 50000 mPa·s, and more preferably no more than 20000 mPa·s.

In particular, the manhole cover, as is customary, is plate-like in appearance and has any appropriate form, for example, rectangular, square, or round. It has two substantially parallel faces, usually an upper face destined to be facing towards the exterior, especially if it is a manhole cover for pits, and a lower face able to rest on a relative seat, generally around the edge thereof, the said seat being part of a frame which is appropriately positioned in correspondence with the opening to be closed. On the surface of these faces there may be protuberances or projections, or depressions or grooves. For example, on the upper face, projections may be provided to render the surface less slippery, and on the lower face, there may be reinforcement, or depressions to reduce the thickness in areas subjected to less stress. In particular, the lower face may have projections in correspondence with the area of the edge intended to rest on the corresponding frame of the seat made in the pit or similar structure, in order to properly distribute the load on the frame and ensure proper closure around the entire perimeter.

According to a preferred aspect of the invention, the fibres, of any appropriate kind, may be of the carbon, aramid, or, more preferably, glass variety, and the polymerized mass of the finished manhole cover may have a concentration by weight of between 20 and 70%. The length of the fibres should preferably be between 10 and 120 mm, and more preferably between 20 and 80 mm. The length may be chosen in consideration of the thickness of the manhole cover, in particular to prevent agglomerates, inhomogeneity, or undesired preferential orientations occurring due to the shifting of the mass of reagents caused by the closure of the mould. Similar considerations can also be made taking into account the viscosity of the reagents and the presence of fillers. The invention will now be better illustrated by means of the description of a preferred embodiment, provided in the form of a non-limiting example, with the help of the accompanying drawings, in which:

FIGS. 1, 2, and 3 represent schematic views, showing respectively, a plan, a cross section cut along plane II-II of FIG. 1, and a bottom-up view of a manhole cover obtainable according to the present invention;

FIG. 4 represents a schematic view of the production process in accordance with the present invention;

FIG. 5 represents a schematic view of the production process in accordance with a different aspect of the present invention.

FIGS. 1 and 2 show a manhole cover for pits, according to a particular aspect of the present invention.

The manhole cover is plate-like and appropriately shaped, as mentioned. In the case shown, the said cover is square-shaped, which represents the most common type.

The manhole cover 1 has an upper face and a lower face 2, which are substantially mutually parallel, and their distance apart is the thickness, which is over 10 mm. As seen, the process according to the invention allows the manufacture of the manhole covers with excellent results as regards homogeneity, mechanical characteristics, and reproducibility of results, even with the thicknesses stated above, which are necessary for parts of that type; it has been found that these results can also be obtained in the construction of manhole covers with thicknesses of over 10 mm, and even with thicknesses up to 80 mm, which constitutes a particular aspect of the present invention. These thickness values should concern at least ⅓ of the surface of the faces, and preferably at least 40% of the said surface. According to one possible aspect of the invention, the thickness of substantially all of the said areas is, however, at least 10 mm. There may be holes, for example, holes that run through the thickness of the manhole covers, designed for the application of extractors, as already commonly occurs in the case of manhole covers according to the commonly known technique. There may by reliefs or channelling on the faces; for example on the upper face, there may be writing or formations in relief 3 and 4 designed to foster friction with feet or soles to prevent slipping, the said writing or formations being similar in size to those which are customary for these structures.

Lighter areas may be provided, with reduced thickness with respect to the rest of the article, such as, for example, the appropriately shaped indentation 5 made on the lower face. The manhole cover resting area on the lower face is constituted of a border 6 with a minimum width of 5 mm, which can be in relief. It can be continuous or discontinuous around the edge of the lower face, depending on requirements, and can have an appropriate section, for example, a semi-circular section, as shown in FIG. 3, to allow adequate deformation and appropriate load distribution, preventing undistributed stress caused by irregular deformation of the manhole cover under load, and improving the seal. There may be teeth 7 present, designed to engage with the indentations in the frame of the pit and act as a hinge point during the early stages of manhole cover lifting, or simply as a guide for correct placement of the manhole cover, and any other type of known structure, commonly used in this field.

The manhole cover is suitable for pit structures of a commonly known type, with frames made of commonly used materials, such as metal, reinforced or non-reinforced plastics, including the types used in the manufacturing of the manhole cover. The frame may, if desired, also be manufactured in different colours. In particular, if the frame is plastic, it can be appropriately coloured through the introduction of pigments into the mixture to be sent to the mould. The colour may perform the function of identifying different characteristics, such as the intended use of the pit, the manufacturer, or otherwise. According to a preferred aspect of the invention, the manhole cover manufacturing process may include manufacturing of the mixture, introduction of the fibre, and injection into the mould using technology known as LFI (i.e. “long fibre injection”) or InterWet. This technology involves measuring out and sending the reagents (preferably polyurethane reagents) to a suitable mixing head and sending the reinforcement fibre, in the form of one or more continuous filaments (called roving), to a cutter that continuously produces fibre flakes cut to a predetermined length.

The cut fibre, which is extracted by a Venturi system inside the mixing head, encounters the mixed reagents and is impregnated therewith.

A homogeneous mixture of fibre and resin is issued from the mixing head and distributed over the surface of the mould, following an optimised path.

The fibre/resin weight ratio is set according to the desired mechanical properties.

A system of air jets diverts the path of the mixture, in flight, thereby allowing modulation of the distribution, to create large, thin or narrow, thick layers of fibre and resin.

The length of the cut and the instantaneous flow rate of the fibre can be varied in real time to create zones of the article with different mechanical properties.

It is also possible to interrupt the flow of fibre, in order to spray solely polyurethane, or to interrupt the flow of polyurethane by casting solely fibre if desired, or by varying the fibre and polymer proportions during different phases.

The fibre, if necessary, with the surface thereof treated with appropriate finishes to render it compatible with the resin, comes in the form of reels. The continuous roving is conveyed, if necessary with the help of air jets, to the cutter via rigid pipes and flexible springs to follow the movement of the spray head.

Appropriate sensors monitor the presence of the roving, correct unwinding of the reels and any clogging of the cutter.

If a problem is encountered, the system immediately stops the application cycle.

This technology is of a kind used commonly in the production of vehicle parts, such as dashboards, bumpers, rear window shelves, exterior panelling for bodywork on industrial and earth-moving vehicles and impact-resistant panels. The articles in whose production this technology is used generally have limited thicknesses and are not made to withstand high bending loads; to increase resistance to these types of stress, they are made with ribbing, whose extension is such that they are not possible in a product as compact as a manhole cover. However, the bending resistance in these cases is not comparable to that required by legislation on manhole covers. It has now been found that it is possible, with this type of technology, to manufacture manhole covers which have necessarily higher thicknesses without generating fibre distribution problems, even with very high fibre lengths and concentrations, such as those mentioned above, which are much higher than those commonly used in the known applications. Moreover, it has been found that these manhole covers have a mechanical resistance, especially to bending loads, tailored to needs, which envisage much higher values than those usually required of articles in the usual fields of application for this technology.

A process according to the present invention shall now be outlined with reference to FIG. 4, in which this technology is applied. Appropriate sources 8 and 9, respectively, supply the polyol and isocyanate, i.e. the two components of the polyurethane matrix, through lines 12 and 13, to the mixing head 11, which homogenises the two components; the reinforcing fibre, coming from one or more reels 14, is fed to the cutter 19, where it is cut and extracted continuously by the device 10 inside the mixing head, where it encounters the polyurethane mixture and is impregnated therewith. The fibre reel, especially if a polyurethane material is used, can be appropriately finished; this pre-treatment renders it compatible the fibre with the polymer. The fibre is usually available on the market already finished. Other components, such as initiators or catalysts or mineral fillers, such as calcium carbonate, for example, may be pre-mixed with one of the reagents or be appropriately introduced. The mixing head 11, which may or may not be in correspondence with the extraction device, is also capable of spraying the mixture into the mould 17, designed to be closed by an appropriate counter-mould. According to a possible aspect of the invention, the mixing/spraying head can be fixed. According to another aspect of the invention, it can be movable, for example mounted on a robotic arm 16, so as to achieve a pre-distribution of the material within the mould before closing the counter-mould.

The technology described above is particularly preferred because of the results obtained in terms of distribution and homogeneity. However, other types of technology are possible, for example, the fibre can also be mixed with the reagents (preferably polyurethane reagents) immediately downstream of the mixing head, for example, according to a technology known as CMS PUR (Composite Spray Moulding) or Outerwet.

With reference to FIG. 5, these technologies involve measuring out and sending the polyurethane reagents (polyol and isocyanate) to a suitable mixing head 11 and sending the reinforcement fibreglass from the reels (always in the form of one or more continuous filaments, a.k.a. roving), to a cutter 19 which produces continuous fibre flakes cut to a predetermined to length.

In this case, a spraying process is used to cast the fibre-enriched mixture. The polyurethane mixture is sprayed directly into the open mould and, by means of an appropriate introduction device 10, the pre-cut fibreglass encounters the jet of reagents mixed at the mixing and spraying head output and is impregnated therewith in mid air, before reaching the surface of the mould.

The polyurethane and fibreglass mixture thus formed is distributed over the surface of the open mould, following an optimised path, as per the technology solution described earlier.

Also with this technology, the measuring and spraying system allows maximum production flexibility by allowing the casting of different layers. In particular, it is possible, for example, to interrupt the supply of pre-cut fibreglass and cast solely a layer of polyurethane. Combinations of layers with different formulations can be adapted to the specific needs of the finished product.

According to a preferred aspect of the invention and with reference to FIGS. 4 and 5, in any case, the mixture is introduced into a mould 17, shaped in correspondence with the upper face of the manhole cover and the mould is then closed by a counter-mould shaped in correspondence with the lower face. This allows better distribution, especially with manhole cover forms such as those exemplified, and evacuation of the air from the mould when it is closed, especially in the presence of projections on the upper face and the indentation on the lower face.

It is also possible, and constitutes a preferred aspect of the invention, to spray a first layer devoid of fibre, or with a lower concentration of fibre, preferably of a thickness of between 0.5 and 2 mm, into the mould with the shape corresponding to the upper face and then supply the rest of the material with fibre. In this way, a manhole cover is manufactured with the upper face devoid of fibre, which is desirable for aesthetic reasons and for the material's surface abrasion resistance. In this way, the need for coatings or other surface treatments is eliminated.

The viscosity of the mixture and the setting time are parameters that can be adjusted by choosing different reagents types, which should be chosen taking into account factors such as the need to avoid creating preferential orientations in the fibres, and the need to avoid entrapment of bubbles in the mould

The mould and the counter-mould can have appropriate gas flow release systems, such as appropriately positioned holes in the counter-mould. The invention will now be better illustrated by means of the following preferred but not exclusive examples of embodiments.

EXAMPLE 1

Using a Cannon FPL InterWet 24 head, 120 g/s of polyol and isocyanate mixture (with an isocyanate/polyol ratio of 1.1/1), with the isocyanate component having a titre of to 31.5% NCO and the polyol component having OH number 420 mg KOH/g, with 80 g/s of glass fibre (Owens Corning 900A X3 4800N ADVANTEX) cut to 75 mm, were sprayed for 16.7 s into an open mould, heated to 65° C. A distribution phase was carried out as stated in the following section, entitled fibre distribution. The mould was closed in the press with a closing force of 100 tons. The moulding time was 7 min, after which the mould was opened and the piece extracted. After removal of the sprue, the final weight of the piece amounted to 3076 g. The fibreglass content, measured at several points of the finished piece, averaged 40%. The piece thus obtained, with a side measuring 335 mm, suitable for a standard 400*400 mm frame, underwent testing to determine residual deformation and the maximum load, which was 125 kN, as required by standard EN124 for class B125 parts: the residual deflection amounted to 0.87 mm, against the specification limit of 6.13 mm envisaged for a piece of this size; following application of the maximum load, there were no cracks or defects in the piece visible to the naked eye.

EXAMPLE 2

Using a Cannon FPL InterWet 24 head, 120 g/s of polyol and isocyanate mixture (with an isocyanate/polyol ratio of 1.1/1), with the isocyanate component having a titre of to 31.5% NCO and the polyol component having OH number 420 mg KOH/g, with 80 g/s of glass fibre (Owens Corning 900A X3 4800N ADVANTEX) cut to 50 mm were sprayed for 17.3 s into an open mould, heated to 65° C. A distribution phase was carried out as stated in the following section, entitled fibre distribution. The mould was closed in the press with a closing force of 100 tons. The moulding time was 7 min, after which the mould was opened and the piece extracted. After removal of the sprue, the final weight of the piece amounted to 3185 g. The fibreglass content, measured at several points of the finished piece, averaged 40%. The piece thus obtained, with a side measuring 335 mm, suitable for a standard 400*400 mm frame, underwent testing to determine residual deformation and the maximum load, which was 125 kN, as required by standard EN124 for class B125 parts: the residual deformation amounted to 0.84 mm, against the specification limit of 6.13 mm envisaged for a piece of this size; following application of the maximum load, there were no cracks or defects in the piece visible to the naked eye. The load values shown in the above tests are those envisaged by the aforesaid legislation. It should be noted that the aforesaid pieces withstood, without presenting detectable cracks or defects, even when subjected to loads of 180 kN.

Fibre Distribution

In the first phase, a layer of polyurethane without fibre is distributed for about 2 seconds. The polyurethane is sprayed on so as to cover a large area.

The mixing head moves horizontally at 250 mm/s along a square path which is concentric to the mould; the path is sufficiently sized and of sufficient height to allow complete coverage of the mould bottom.

Once the path is complete, the head moves up to a higher level and activates the fibreglass supply.

The head completes a further two laps around the casting path at a speed of 80-85 mm/s, for a duration of 15-16 seconds.

At the end, the head stops the supply of fibre and polyurethane.

Distribution of the polyurethane and fibreglass in two laps completed quickly is more homogeneous than distribution in a single lap at half speed. 

1. A process for manufacturing a manhole cover having a thickness of at least 10 mm, comprising the generation of a mixture of reagents able to generate a polymeric thermosetting material, the mixing of fibres of suitable reinforcement material, the introduction into an open mould of the so obtained mixture, the closure of the mould with a suitable counter-mould, the hardening of said mixture, the opening of the mould and the removal of the obtained manhole cover.
 2. The process according to claim 1, wherein the manhole cover is made of polyurethane and the fibre is glass-fibre.
 3. The process according to any of preceding claims, wherein the fibre is between 20 and 70% of the finished product.
 4. The process according to any of preceding claims, wherein the length of the fibres can be preferably comprised between 10 and 120 mm, optionally between 20 20 and 80 mm.
 5. The process according to any of preceding claims, wherein a manhole cover is made, able to withstand the application of a load of at least 100 kN to a face called upper face, with the manhole cover laid on the opposite face, called lower face, with the border (measuring a minimum of 5 mm) around the edge of the said lower face resting in position.
 6. The process according to any of preceding claims, wherein the fibre is cut and sucked continuously, by a Venturi tube or similar device, into a mixing head, where it meets the mixture of reagents and is impregnated with them before being introduced into the mould.
 7. The process according to any of claims 1 to 5, wherein the fibre is introduced into said mixture after a spraying head, being impregnated with said mixture in air.
 8. The process according to any of preceding claims, wherein the mixture is sprayed in a mould having a shape corresponding to an upper face of the manhole cover, and the counter-mould having a shape corresponding to a lower face, and in said mould a first mixing layer is spayed, which has no fibres or has a reduced concentration of fibres with respect to the rest, at the upper face of the manhole cover.
 9. A manhole cover made of a thermosetting plastic material, having a thickness of at least 10 mm, comprising a plurality of fibres of reinforcement material, uniformly dispersed in the volume of the thermosetting material.
 10. The manhole cover according to claim 9, obtainable by means of a process according to any of claims 1 to
 8. 